Pulse trains of electromagnetic radiation, typically in the visible or infrared part of the electromagnetic spectrum, find many uses in science and technology. For instance, almost all existing or planned optical communication systems are of the digital type and thus employ pulses of electromagnetic radiation. Other applications of such pulse trains are, inter alia, in optical radar, optical ranging, optoacoustic spectroscopy, optical computing, and reaction rate studies.
The prior art knows many techniques for forming optical pulses, such as rotating slotted discs, pulsed lasers, diodes, or flash lamps. However, most older prior art techniques cannot conveniently and inexpensively produce a high repetition rate sequence of very short pulses, e.g., with repetition rates of more than 10.sup.9 sec.sup.-, and with pulse widths less than 10.sup.-9 sec.
Recently some methods have been developed that are capable of producing exceedingly short pulses, in the picosecond, and even femtosecond, range. These include pulse compression methods and the soliton laser method. Such ultrashort pulses are of great scientific interest, since they permit previously unattainable time resolution in a number of scientific experiments. However, these methods do not easily lend themselves to the production of pulses having very high repetition rate, e.g., in excess of 10.sup.9 sec.sup.-1.
It has recently been discovered that it is possible to transmit information at very high bit rates, of the order of tens or even hundreds of Gbits/s, over single mode optical fiber if shape-preserving pulses, generally referred to as optical solitons, are used. See, for instance, U.S. Pat. Nos. 4,368,543, 4,406,516, and 4,558,921, all co-assigned with this, and incorporated herein by reference.
However, in order to utilize the high data transmission rate of which soliton-based systems are capable, it is necessary to have available pulse-generating means that can produce narrow optical pulses at a very high repetition rate. U.S. patent application Ser. No. 602,694, ('694) filed Apr. 23, 1984 by A. Hasegawa, co-assigned with this, and incorporated herein by reference, discloses that optical pulses can be produced by means of the so-called induced modulational instability of cw (continuous wave) radiation in an appropriate optical medium, e.g., single mode optical fiber.
The modulational instability has previously been used to produce tunable coherent infrared and far infrared electromagnetic radiation. See, U.S. Pat. No. 4,255,017 ('017), issued Mar. 10, 1981, to A. Hasegawa, co-assigned with this, and A. Hasegawa and W. F. Brinkman, IEEE Journal of Quantum Electronics, Vol. QE-16(7), pp. 694-697. The method of the '017 patent comprises injection of unmodulated cw radiation of a given wavelength, the carrier, into single mode optical fiber, the carrier wavelength chosen to lie within the regime of anomalous dispersion of the fiber core material. Due to the combined effect of the anomalous dispersion and the nonlinear Kerr effect, side bands of the carrier are produced; in other words, amplitude modulation of the injected unmodulated carrier wave results. Rectification of the modulated carrier yields an output signal of a frequency proportional to the square root of the power in the carrier wave.
The '694 application teaches that a pulse train of predetermined pulse spacing .tau..sub.M (pulse repetition rate 1/.tau..sub.M) can be created by coupling intensity modulated cw radiation of carrier wavelength .lambda..sub.o into a nonlinear transmission medium having an anomalous dispersion region that includes .lambda..sub.o. Pulse formation occurs through interaction of the electromagnetic radiation with the transmission medium. The nonlinearity of the medium produces self-steepening of the amplitude peaks, and the spacing between the intensity peaks of the injected cw radiation determines the spacing between the pulses.
In order to practice the pulse formation method of the '694 patent application it is necessary to modulate the carrier radiation intensity. In some important applications it is necessary to have repetition rates as high as 10.sup.10, or even 10.sup.11 sec.sup.-1 or more. Modulation at such high modulation frequencies is currently at best difficult to achieve, since electronics capable of operation at such high frequencies is not available. A method for forming a train of optical pulses by means of the induced modulational instability that does not require the use of high speed electronics would thus be of considerable interest. This application discloses such a method.